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Old 14th January 2013, 11:14 AM   #61
EUVL is offline EUVL  Europe
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> the 844 as-is is much better than almost all discrete circuitry

Except when you want to use it as a BJT CEN and wants lower input impedance than 65 ohm ?



(PS I like the 844 as well, especially as in LC's phono.)


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Old 14th January 2013, 07:43 PM   #62
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Here is what works great for my PCM1704, if your doing a sub for another I/V opamp, don't forget to disconnect the feedback loop around the old opamp.
And give thanks to Pedja Rogic for letting this out of the bag, as I think it would be hard pressed for even a discrete I/V to better the sound this can give. And kudos Barrie Gilbert for designing the AD844 and abraxilito for the dac dc offset nulling circuit
Cheers George
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Last edited by georgehifi; 14th January 2013 at 07:53 PM.
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Old 14th January 2013, 07:48 PM   #63
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And for TDA5141. Again don't forget to remove all feedback from the old opamp, and you probably could use the simpler dac offset nuller that abraxilito did for the PCM1704..
Cheers George
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Last edited by georgehifi; 14th January 2013 at 07:52 PM.
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Old 14th January 2013, 07:49 PM   #64
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If I understood correctly instead the 15v psu + 2sk170 (2sk369) + VR1 + C1 can't be used, I must use the 2nd pic with the +-5V, right? with 20K preset I have to ajust 0VDC at pin 2 of AD844, right?
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Old 14th January 2013, 07:56 PM   #65
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I leave that for others to answer as I have only used the simpler abraxilito method.

Cheers George
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Old 14th January 2013, 09:06 PM   #66
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Quote:
Originally Posted by EUVL View Post
> the 844 as-is is much better than almost all discrete circuitry

Except when you want to use it as a BJT CEN and wants lower input impedance than 65 ohm ?



(PS I like the 844 as well, especially as in LC's phono.)


Patrick
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Old 15th January 2013, 12:54 AM   #67
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Originally Posted by abraxalito View Post
I suggest you read his reports a bit closer - he only got the issues when the reg was powering something digital from my reading of it. If it had been stability he could have fixed that up, he knows enough about Bode plots I reckon - in a reg its the stability of the whole loop, not just the opamp.
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Walt and I also had issues with 797 stability in our 1994 regulator series. I tried to convince Walt to use another opamp which he eventually agreed to.
The 'touchiness' of the 797 was related to the internal neutralisation, which is a form of positive feedback which can, depending on the feedback loop characteristics, cause instability. That doesn't mean its not a good chip - its a very good device, but you must know what you are doing.

BTW I just read earlier posts about people biasing the Tz node of the AD844 up to 10mA to force it into class A. Please be aware that at those currents, the Tz node is quite nonlinear, causes gross distortion and may also exhibit slew rate limiting. Don't get hung up on class A too much - the 844 as-is is much better than almost all discrete circuitry.

jan
ironic much?

yes it was that series of articles I remembered it from, related to the superregulators.

in no way am I criticising Walt, or Scott's work, clearly they both know what they are doing..... I have had excellent results with the AD797, even at unity gain in Flea type regs, its one of my favorite opamps, perhaps THE favorite opamp.

It does require a bit of babying at low gain though in my experience and I believe its unity gain stability stamp should come with a disclaimer, because it is used in ways it is not really suitable for, by opamp rollers and designers alike because of this.

Jan, you mean pin 8?, yes the layout and component choice regarding this pin's use are critical, as well as being quite important to squeezing the most out of it. either use it properly or not at all IMO
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Old 15th January 2013, 01:09 AM   #68
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What a pity that I'm totally missing the irony there, despite having a fairly refined taste for it.

Here's a paper which sheds some light on what might well have been the issue that Walt found : RF-induced offset variations in opamp input stages.
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Old 15th January 2013, 01:19 AM   #69
qusp is offline qusp  Australia
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I suggest you read his reports a bit closer - he only got the issues when the reg was powering something digital from my reading of it. If it had been stability he could have fixed that up, he knows enough about Bode plots I reckon - in a reg its the stability of the whole loop, not just the opamp.
Quote:
Walt and I also had issues with 797 stability in our 1994 regulator series
that, is ironic.

perhaps your taste for it is not as refined as you think?

of course it was stability of the whole loop, isnt it always? but it was stability of the AD797 in that whole loop, so it was discarded and not recommended, replaced by AD825 if I remember correctly.

he could most likely have worked with it and fixed it in situ, but he was writing an article for others to implement and clearly there were a few issues that made it hard to recommend under this use case.

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Old 15th January 2013, 01:21 AM   #70
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You're missing important detail there because of not taking my advice to read his reports closer. I'll leave you to figure out what you're missing Hint - the date I was talking about wasn't 1994.
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